The present invention relates to an automatic drift control circuit which automatically corrects drift of a multi-level demodulated signal.
Demodulation of a received multi-level QAM (Quadrature Amplitude Modulation) signal with a recovered carrier involves converting it to a digital signal by level decision. If the demodulated signal, if the demodulated output signal level deviates from the correct level, it can no longer be converted into a correct digital signal and therefore it must be corrected automatically. For this purpose, an automatic drift control circuit is used.
The demodulated signal is amplified up to a specified level by an amplifier, input to an A/D converter through an adder and then converted into a digital signal consisting of a plurality of bits. For example, when a 256-level QAM signal is demodulated and converted into a digital signal, a 4-bit data structure can be obtained since the I and Q channels respectively become 16-level data. A single bit position, namely the 5th bit from the most significant bit (MSB) indicates whether or not the input signal is higher or lower than the decided level among the specified 16 levels. Therefore, it is used as an error signal and applied to an integrator for integration. The integrated output therefrom is applied to the adder and thereby drift of demodulated signal is corrected.
When drift of the demodulated multi-level signal to be input to the A/D converter is within a range of a decision level for a given error signal, the drift can be corrected and a multi-level signal can be decided correctly. But, if level variation of the received signal becomes large due to fading, etc. and thereby causes drift of the multi-level signal become large, the demodulated signal exceeds a threshold level for data decision in the A/D converter and polarity of the error signal is thereby inverted causing a pseudo pulling condition. Namely, the signal is pulled under the condition that the level of the multi-level signal is shifted upward or downward for the input level range of the A/D converter. In such a pseudo pulling condition, correct data cannot be obtained.